Plasma system and method for anisotropically etching structures into a substrate

ABSTRACT

A method and a plasma system are provided for anisotropically etching structures into a substrate positioned in an etching chamber, e.g., structures defined using an etching mask in a silicon substrate, using a plasma. For this purpose, the etching chamber is supplied at least intermittently with an etching gas and at least intermittently with a passivation gas, the passivation gas being supplied to the etching chamber in cycles having a time period between 0.05 second and 1 second. In the plasma system, in addition to a plasma source, via which the plasma acting on the substrate may be produced, an arrangement is provided for at least temporary supply of the etching gas and at least temporary supply of the passivation gas to the etching chamber, which arrangement is designed in such a way that the passivation gas may be supplied to the etching chamber in cycles having a time period between 0.05 second and 1 second.

FIELD OF THE INVENTION

The present invention relates to a plasma system and a method foranisotropically etching structures, in particular structures definedusing an etching mask, into a silicon substrate, using a plasma,according to the definition of the species in the independent claims.

BACKGROUND INFORMATION

A method for high-rate plasma etching of silicon is described, e.g., inpublished German patent document DE 42 41.045, etching being alternatedwith deposition of a Teflon-like polymer on the side walls of etchedstructures, which protects these walls from an etching operation duringthe subsequent etching steps. Gases which provide fluorine radicals inplasma, such as SF₆, NF₃, or ClF₃, are used as the etching gases. Gaseswhich provide Teflon-forming monomers in plasma, such as C₄F₈ or C₃F₆,are used as the passivation gases. This method allows etching rates ofup to 20 μm/minute with excellent structure precision and selectivityeven in regard to simple mask materials such as photoresist or SiO₂.

In the method described in published German patent document DE 42 41045, comparatively short deposition steps and/or passivation gas stepsand longer lasting etching steps are used in order to achieve thehighest possible etching rate. Formulas in which passivation gas stepsof 3 to 5 seconds each and etching steps of 10 to 12 seconds eachalternate with one another are typically used in connection withinductively coupled plasma sources. In the case of shorter passivationgas steps, it becomes increasingly more difficult to reproduce them withthe required precision over a very large number of cycles.

An object of the present invention is to provide a plasma system and amethod which make it possible to achieve a higher etching rate whenanisotropically etching silicon as the substrate, in comparison to theprior art, while simultaneously providing greater profile control andalso greater mask selectivity.

SUMMARY

The method and the plasma system according to the present invention havethe advantage of a higher etching rate, e.g., when anisotropicallyetching silicon as a substrate, while simultaneously providing greaterprofile control and additionally greater mask selectivity.

It is advantageous that the achievable etching rate and the structureprecision increasingly improve with shorter cycle times of thepassivation gas cycles, so that the very short passivation gas cyclesused according to the present invention approach an “optimum” process inwhich no interruptions by passivation gas use and/or no passivationsteps are necessary at all; instead, etching is performed uninterrupted.However, such an “optimum” process would lead to undesired isotropicetching instead of anisotropic etching, while the process according tothe present invention, in spite of the short passivation gas cycles,still allows anisotropic etching of structures.

The plasma system and the method according to the present invention thusallow extensive approximation of an “optimum process” while maintainingthe anisotropy of the etching and high profile control and maskselectivity.

Furthermore; the plasma system according to the present invention hasthe advantage that it may be built on a typical plasma system having aninductively coupled plasma source, for example, so that no significantsystem investments are necessary and/or existing plasma systems may beappropriately retrofitted without significant additional expenditure.

It is advantageous that the method according to the present inventionmay be implemented through improvement of the method for anisotropicetching of silicon according to published German patent document DE 4241 045 or may be integrated therein, and thereby nearly uninterruptedplasma etching, which is distinguished by particularly high etchingrates with particularly good structure precision and minimal undercutsor wall roughness, is achievable in this case.

Furthermore, it is advantageous if, in the method according to thepresent invention based on an improvement of the method according topublished German patent document DE 42 41 045, a passivation gas whichprovides the strongest possible passivation and Teflon-forming monomersis used as the passivation gas. In addition to C₄F₈ or C₃F₆,hydrofluorocarbons having an even lower fluorine to carbon ratio, suchas C₄F₆ (hexafluoro-1,3-butadiene) or C₅F₈ (octafluoro-1,3-pentadiene),and even C₂H₂F₂ (difluoroethylene), are suitable for this purpose. C₄F₆,which forms polymers particularly efficiently, is particularlyadvantageous. These gases may be removed from the buffer tank during thepassivation steps with gas flow and/or material quantity which isreduced in relation to C₄F₈ or C₃F₆.

With the passivation gases C₄F₆ or C₅F₈, as well as with the gases C₄F₈or C₃F₆, a more rapid polymer deposition from the plasma advantageouslyoccurs overall with otherwise comparable plasma characteristic data, thedeposited polymer additionally being denser and, due to the lowerfluorine to carbon ratio of these gases, also significantly morestrongly cross-linked. In addition, the deposited polymer is moreresistant to etching erosion because of a higher carbon content.

When anisotropically etching silicon as described in published Germanpatent document DE 42 41 045, for example, in addition to the fluorineradical concentration available in the etching step, the efficiency ofthe buildup of a side wall polymer film as a protective film and itsresistance in the following etching step play a decisive role in theprocess performance, and hence significant improvement potential resultsdirectly therefrom.

In particular, the improved properties of the polymer deposited as aside wall film in the passivation steps in regard to density,cross-linking, higher carbon content, and increased resistance toetching erosion allow significantly shorter passivation gas cycle timesof well below 1 second, for example, 100 ms to 500 ms, and alsocomparatively significantly lengthened etching gas cycle times fromapproximately 1 second up to 20 seconds to 30 seconds, i.e., the ratioof etching time to passivation time shifts significantly in favor of theetching time to values of 10:1 to 30:1 or even more. The higherchronological proportion of the etching steps in the total process timeleads directly to correspondingly higher etching rates.

Additional factors contributing to higher etching rates are the highcarbon content of the side wall polymer films, their strongercross-linking, and therefore greater resistance to erosion.

Since, because of the very short passivation gas cycle times and theused passivation gases C₄F₆ or C₅F₈ or C₂H₂F₂, less side wall polymermaterial, in comparison with the passivation gases C₄F₈ or C₃F₆ isinitially eroded during the etching steps, which are isotropic per se,and redeposited in lower-lying regions of the resulting structuresand/or the side walls of the resulting trenches of the etching step, butbecause of the higher resistance of the produced Teflon-like side wallpolymer films, a sufficient local passivation effect and localanisotropy of the etching step connected therewith are still achieved,so that less fluorine is lost through interaction with polymer materialsand/or passivation gas components transported by ions during the etchingprocess. The achieved stronger C—C cross-linking also helps in thiscase, since fluorine radicals attack internal C—C bonds less than freeexternal C bonds. This also increases the efficiency of the overalletching process.

The dynamics of the side wall polymer are known to have a largeinfluence on the net quantity of available free fluorine radicals. Ahigher fluorine radical concentration, which is active in relation tosilicon, together with greater permissible etching cycle durationsand/or etching gas cycle times in relation to the passivation cycledurations and/or the passivation gas cycle times, thus significantlyincrease the achievable etching rates.

Finally, the mask selectivity is advantageously improved in accordancewith the present invention in that the passivation coatings deposited onthe mask used are also more resistant than in the known art due to themechanisms described, and therefore the substrate masking, e.g., themasking of a silicon wafer, is passivated particularly effectivelyduring the etching process, in the case of a photoresist mask, forexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a plasma system according to a firstexemplary embodiment.

FIG. 2 shows a detailed illustration of a modified gas supply controlleraccording to a second exemplary embodiment.

DETAILED DESCRIPTION

FIG. 1 shows a plasma system 5 having an etching chamber 12 and a plasmasource 19. In particular, an ion discriminator is located below plasmasource 19 and has two coils which have a current flowing through them inopposite directions during operation, an upper coil 14 and a lower coil13, and a drift zone, which is provided by a “spacer” or a spacer ring,for propagation of a plasma 22, produced in etching chamber 12 in theregion of plasma source 19, in the direction of a substrate 21, such asa silicon wafer, which is located on a substrate electrode 20.Furthermore, substrate electrode 20 is connected via a first impedancematching device 11 (first “matchbox”) to a substrate bias generator 10.Plasma source 19 is an inductively coupled plasma source having a coil18, which is connected via a second impedance matching device (second“matchbox”) to a coil generator 15. Finally, the etching chamber has ahigh-performance pump device 31, via which etching chamber 12 may beevacuated.

Plasma system 5 according to the present invention has a modified gassupply device 32 of etching chamber 12. A feed line 23 is provided forthis purpose, which is as short as possible, e.g., not longer than 20cm, and which discharges into etching chamber 12, and is connected to anetching gas line 26 and a separate, independent passivation gas line 25.Furthermore, an etching gas valve 27 is provided upstream from thedischarge of etching gas line 26 into feed line 23, and apassivation-gas valve 28 is provided upstream from the discharge ofpassivation gas line 25 into feed line 23. Finally, a buffer tank 24 isinserted into passivation gas line 25 upstream from passivation gasvalve 28. In addition, a buffer tank (not shown in FIG. 1), whichbuffers the etching gas supply during the passivation gas cycles, i.e.,when etching gas valve 27 is closed, may be provided in etching gas line26 upstream from etching gas valve 27.

Valves 27, 28 may be positioned as close as possible to etching chamber12, i.e., the gas lines which follow subsequent valves 27, 28 are asshort as possible. Furthermore, buffer tank 24 is also to be positioneddirectly upstream from passivation gas valve 28. In addition, feed line23 may be dispensed with completely if etching gas line 26 andpassivation gas line 25 discharge directly into etching chamber 12 viatwo assigned inlet openings. In this case, valves 27, 28 may be eachlocated directly upstream from these inlet openings on etching chamber12.

Finally, a programmable control unit 17, via which valves 27, 28 may beactuated, is provided in FIG. 1. In addition, two mass flow regulators(not shown in FIG. 1) are provided, which are positioned upstream fromvalves 27 and 28, respectively, and are assigned to etching gas line 26and passivation gas line 25, respectively, or are integrated therein.Control unit 17 may also be connected to these mass flow regulators, asis explained in FIG. 2 in connection with a second exemplary embodiment.In this case, etching gas line 26 is assigned a first mass flowregulator 29 and passivation gas line 27 is assigned a second mass flowregulator 30, which again may be positioned as close as possible toetching chamber 12. Alternatively, it is also possible to place massflow regulators 29, 30 distal from etching chamber 12, in a “gas box,”for example.

Control unit 17 may be integrated into the process sequence controllerof plasma system 5 as a hardware component or may be a part of theprocess sequence controller, i.e., the software code, for example.

Buffer tank 24 may have a volume from 0.1 L to 1 L, for example, 0.5 L,and may be made of stainless steel having electropolished internalwalls. If a buffer tank is also provided in etching gas line 26 upstreamfrom etching gas valve 27, it also has an analogous design.

Etching gas valve 27 is designed as a “normally open” valve duringoperation of plasma system 5, while passivation gas valve 28 is designedas a “normally closed” valve. Valves 27, 28 discharge, as explained, asdirectly as possible into etching chamber 12, and either separate shortgas lines having the largest possible cross section and a length of atmost 20 cm, or shared short feed line 23, also having the largestpossible cross section, may be provided for the outlets of etching gasvalve 27 and passivation gas valve 28. The connection of buffer tank 24to passivation gas valve 28 is also to have a length of less than 20 cmand is to be designed to have the largest possible cross section. Inaddition, it is possible to combine etching gas valve 27 and passivationgas valve 28 into a single changeover valve, which connects etching gasline 26 in the rest state and passivation gas line 25 in the activatedstate through to etching chamber 12. This results in synchronization ofthe opening and closing operations for lines 25, 26 in a simple way.

Using plasma system 5, structures which are defined laterally with theaid of an etching mask are anisotropically etched in silicon, i.e.,substrate 21, according to published German patent document DE 41 42045, C₄F₆ being used as the passivation gas, for example. For thispurpose, valves 27, 28 are alternately opened and closed. Passivationgas valve 28 and etching gas valve 27 are preferably rapidly switchingvalves, solenoid valves, for example, which may be activated directlyusing a 24 V signal, for example, and have switching times in the rangeof tens of milliseconds. If 24 V are applied in this case, etching gasvalve 27 closes and shuts off the etching gas conducted in etching gasline 26 from etching chamber 12, while passivation gas valve 28 opensand releases the passivation gas conducted in passivation gas line 25 toetching chamber 12. Correspondingly, a changeover valve switches theetching chamber from etching gas to passivation gas, i.e., to buffertank 24, when 24 V voltage is applied. Alternatively, pneumatic valveshaving electrical pilot valves are also conceivable for valves 27, 28,but a slower response would result.

When plasma system 5 is operated in accordance with an etching method asdescribed in published German patent document DE 42 41 045, for example,an etching gas flow of 300 to 1000 sccm SF₆, e.g., 500 sccm SF₆, and apassivation gas flow of 10 to 500 sccm C₄F₆, e.g., 50 sccm to 200 sccmC₄F₆ (sccm=cm³/minute at normal pressure) are used. The power used atinductive plasma source 19 is from 2000 watts to 5500 watts.

As shown in FIG. 2, mass low regulators 29, 30 assigned to etching gasline 26 and the passivation gas line 25, respectively, are each set to afixed gas flow, for example, 500 sccm SF₆ and 100 sccm C₄F₆, in avariation of the method according to the present invention. The etchinggas SF₆ first flows via open valve 27 into etching chamber 12, while thepassivation gas C₄F₆ first fills buffer tank 24 and is prevented fromflowing into etching chamber 12 by closed valve 28. After 5 seconds, forexample, a short pulse is then sent to both valves 27, 28 by controlunit 17, so that valve 27 closes and blocks the further flow of theetching gas into etching chamber 12 for a short period of time from 0.05second to 1 second, e.g., from 0.1 seconds to 0.5 seconds, whilepassivation gas valve 28 opens for this period of time and releases thepathway of the passivation gas into etching chamber 12, so that buffertank 24 empties nearly instantaneously into etching chamber 12. Afterthe preselected cycle time of 0.05 second to 1 second, for example, 0.3second, has elapsed, valve 27 is opened again and valve 28 isaccordingly closed again by appropriate electrical signals from controlunit 17, i.e., the etching gas again flows into etching chamber 12 andthe passivation gas again fills buffer tank 24 until the describedswitching cycle repeats after a further 5 seconds. 1 second to 15seconds, e.g., 2 seconds to 7 seconds, is set as the time intervalbetween the passivation gas cycles, i.e., as the time duration of theetching gas cycles.

Overall, etching is performed the great majority of the time in this wayand the etching procedure is only interrupted during the very shortpassivation gas cycles and a surge of passivation gas is fed to plasmasource 19, so that a thin layer of Teflon passivation is laid over alletched structures and also provides the necessary side wall passivationfor the subsequent etching step.

In the method described above in connection with FIG. 2, mass flowregulators 29, 30 operate continuously. The brief interruption of theaccess to etching chamber 12 is not significant in this case for theetching gas flow and/or it may be captured as needed by the additionalbuffer tank described for the etching gas. The passivation gas flowscontinuously from second mass flow regulator 30 into buffer tank 24,which empties periodically into etching chamber 12 during the very shortpassivation gas cycles. The amount of passivation gas accumulated inbuffer tank 24 before passivation gas valve 28 is opened thus determinesthe amount of side wall passivation which is incorporated in the etchingprocess. For a time interval t₁ between the individual passivation gascycles, which is synonymous with the time period of the etching cyclesor the etching gas cycles in the above-described method, the materialquantity of the passivation gas accumulated in buffer tank 24 is equalto the product of gas flow and this time t₁.

In a second variation of the method according to the present invention,second mass flow regulator 30 is additionally cycled in synchronizationwith the control of valves 27, 28 via control unit 17 according to FIG.2. This avoids the difficulty or disadvantage of the material quantityof the passivation gas which reaches etching chamber 12 during thepassivation gas cycles being scaled directly with the time interval ofthe passivation gas cycles, which corresponds to the etching stepduration, so that every change of the etching step duration or theetching gas cycle time also results in a change of the accumulatedpassivation gas quantity in buffer tank 24.

For example, if the time interval between the passivation gas cycles,i.e., the etching step duration, is halved, the time during which buffertank 24 is filled with passivation gas is correspondingly also halved.In order to correct this halved charging time, the passivation gas flowto be provided by second mass flow regulator 30 must then be doubled inthe process program via control unit 17.

This coupling of etching step duration or etching gas cycle time t₂ andtime interval t₁ of the individual passivation gas cycles, which resultsin a comparatively complex process adaptation for the individual case,is avoided in that second mass flow regulator 30 is switched to beactive via control unit 17 only for a tank charging time t_(L), which isshorter than etching step duration t₂. For this purpose, after theetching gas flow is reintroduced into etching chamber 12, second massflow regulator 30 is switched off for a time span t₂-t_(L), i.e., therequested passivation gas flow is set to 0 during this time. Second massflow regulator 30 is again set to its setpoint gas flow until the end ofthe following passivation gas cycle and/or the beginning of thefollowing etching gas cycle only after waiting time t₂-t_(L) has elapsedafter the beginning of the etching step. In this case, for example, thevalues t₂=5 seconds, t_(L)=² seconds, and the passivation gas flow 200sccm are selected.

However, this method also has the disadvantage that during thepassivation gas cycles, not only the contents of buffer tank 24 flowinto etching chamber 12, but a small amount of passivation gas is alsosubsequently supplied by second mass flow regulator 30 during thepassivation gas cycle. Although the passivation gas cycle is short andthis quantity is correspondingly small, the response behavior ofpassivation gas valve 28 is transferred into the process.

In a third variation of the method according to the present invention,second mass flow regulator 30 is therefore controlled by control unit 17so that the passivation gas flow is only regulated up to its setpointvalue after passivation gas valve 28 is closed and remains there fortank charging time t_(L), which is less than etching step duration t₂ orthe time t₁ between passivation gas cycles, but is regulated back downto 0 for remaining time t₂ or t₁ after t_(L) has passed. In this case,t_(L) is always less than t₁ or t₂. The amount of passivation gas whichflowed into buffer tank 24 during time span t_(L) thus remains trappeduntil passivation gas valve 28 is opened briefly and the storedpassivation gas amount may flow over into etching chamber 12. Duringthis time, second mass flow regulator 30 is still regulated to 0 andthus may not subsequently supply passivation gas. The supplied materialquantity of passivation gas may be set particularly precisely with thismethod control and is determined independently of the switching behaviorof passivation gas valve 28, which makes setting the process easier.

In a fourth variation of the method according to the present invention,at least one of the parameters selected from etching step duration t₂,tank charging time t_(L), etching gas flow, passivation gas flow, and asubstrate bias power coupled into substrate 21 via substrate electrode20 is varied as a function of time so that the process first starts witha high proportion of passivation and the amount of deposited polymer isreduced continuously or in discrete steps during the process.

Although the above-described methods are independent of the details of aspecific plasma source 19 per se, specific boundary conditions are to betaken into consideration. Thus, gas pulse operation, in which asignificant material quantity of a gas suddenly flows into plasma source19 to replace another gas, results in a sudden pressure increase andaltered plasma conditions, e.g., n regard to the gas types and theirelectronegativity.

In addition, for a method according to published German patent documentDE 42 41 045, it is important for plasma 22 always to remain stable andwell “matched” via second impedance matching device 16, i.e., for it notto go out or blink if possible. This requires a plasma source 19 whichis tolerant to process fluctuations. An inductively coupled plasmasource 19 having a high coupled high-frequency power of 3000 watts to5500 watts and an etching chamber having the smallest possible internaldiameter of only 5 cm to 20 cm, e.g., 9 cm to 15 cm, may be used. Thepower per area in the region of plasma source 19 or at the location ofsubstrate 21 is increased in this way by more than one order ofmagnitude to values of more than 5 watts/cm², e.g., 20 watts/cm² to 30watts/cm². A plasma 22 of this type is particularly tolerant to processparameter fluctuations.

In order not to impair the uniformity of the etching results over thesurface of substrate 21 in this case in spite of diminished etchingchamber 12, it is very advantageous to use the magnetic iondiscriminator, e.g., as described in published German patent document DE100 51 831, in combination with a drift zone in plasma system 5. Thisresults in homogenization of the distribution of the neutral radicalsfrom plasma 22 on their pathway from plasma source 19 toward substrate21, while the magnetic ion discriminator ensures homogenization and/orfocusing of the ion beam to substrate 21 and reflection of electrons. Inaddition, the magnetic fields generated via lower coil 13 and upper coil14 also transfer to plasma source 19 and cause increased electrondensity in plasma 22, which is accordingly more robust and tolerant tosudden changes of essential gas parameters such as pressure, gas flow,gas type, and electronegativity.

As an alternative to the variations of the method described above, theuse of valves which change over alternately between supplying etchinggas and passivation gas to etching chamber 12 and a bypass line to afore-vacuum pump (not shown in FIG. 1) may also be implemented. Thisdoes allow the desired shorter cycle times, since then the regulatingspeed of mass flow regulators 29, 30 is no longer decisive for the cycletimes, but rather the closing times of the valves. However, thisprocedure has the disadvantage that a significant proportion of thecostly process gases are lost unused and must be disposed of as exhaustgas. This is true in particular for the passivation gas, which must besupplied to etching chamber 12 only for the shortest possible time, butadvantageously with a correspondingly large gas flow, and during theremaining time, i.e., the etching step duration and/or during thefollowing etching gas cycle, flows out unused via the bypass line. Thismethod is thus possible, but is comparatively costly.

1. A method for anisotropically etching structures into a substratepositioned in an etching chamber, comprising: providing an etching maskon a silicon substrate positioned in the etching chamber; and providingthe etching chamber at least intermittently with an etching gas and atleast intermittently with a passivation gas; wherein the passivation gasis supplied to the etching chamber in cycles each having a time periodbetween 0.05 second and 1 second; wherein the etching gas and thepassivation gas are used alternately during separate etching steps andpassivation steps that are controlled independently of one another, thepassivation gas being supplied to the etching chamber substantially onlyduring the passivation steps, and the etching gas being supplied to theetching chamber substantially only during the etching steps; and theduration of the passivation steps is set to be shorter than the durationof the etching steps by a factor of 10 to 30; wherein a passivation gasline is provided upstream from the etching chamber, a buffer tank islocated along the passivation gas line upstream from the etchingchamber, a passivation gas valve is located downstream from the buffertank and upstream from the etching chamber, and an etching gas line isprovided upstream from the etching chamber; wherein the passivation gasline and the etching gas line one of: a) connect directly into theetching chamber; and b) connect directly into a common feed lineupstream from the etching chamber, wherein the common feed line feedsinto the etching chamber; wherein all of the passivation gas supplied tothe etching chamber passes through the passivation gas line and thebuffer tank; and wherein during the etching steps, the buffer tank isfilled with passivation gas, and during the passivation steps, thebuffer tank is emptied and the passivation gas formerly in the buffertank flows into the etching chamber.
 2. The method as recited in claim1, wherein the cycles have an identical time period between 0.1 secondand 0.5 second.
 3. The method as recited in claim 1, wherein the etchinggas is also supplied to the etching chamber in cycles each having a timeperiod between 1 second and 15 seconds.
 4. The method as recited inclaim 3, wherein the anisotropic etching is performed in separate,sequentially alternating etching and passivation steps, and wherein aTeflon®-like polymer is applied to at least one lateral delimitation ofthe etched structures with the aid of the passivation gas during thepassivation steps, the polymer being at least partially eroded duringthe etching steps following the passivation steps and being redepositedin lower regions of the etched structures.
 5. The method as recited inclaim 3, wherein a high-density plasma having at least 10¹² reactivespecies/cm³ is provided for the etching steps, and wherein pulsed ionbombardment of the substrate having an ion energy from 1 eV to 100 eV inone of continuous wave operation and averaged over time is performed atleast intermittently during the etching steps.
 6. The method as recitedin claim 1, wherein the passivation gas includes at least one of C₄F₈,C₃F₆, C₄F₆, C₅F₈, and C₂H₂F₂.
 7. The method as recited in claim 1,wherein the amount of the passivation gas used during each of theindividual passivation steps is reduced one of continuously and in stepsas etching progresses.
 8. The method as recited in claim 1, wherein thecommon feed line has a length of less than 20 cm.
 9. A plasma system foranisotropically etching structures into a substrate, comprising: anetching chamber for accommodating the substrate, wherein the substrateis positioned on a substrate electrode located within the etchingchamber; a plasma source for producing a plasma acting on the substrate;and a supply arrangement for at least intermittently supplying anetching gas and at least intermittently supplying a passivation gas tothe etching chamber; a passivation gas line provided upstream from theetching chamber; a buffer tank located along the passivation gas lineupstream from the etching chamber; a passivation gas valve locateddownstream from the buffer tank and upstream from the etching chamber;and an etching gas line, wherein the passivation gas line and theetching gas line one of: a) connect directly into the etching chamber;and b) connect directly into a common feed line upstream from theetching chamber, wherein the common feed line feeds into the etchingchamber; wherein all of the passivation gas supplied to the etchingchamber passes through the passivation gas line and the buffer tank; andwherein during the etching steps, the buffer tank is filled withpassivation gas, and during the passivation steps, the buffer tank isemptied and the passivation gas formerly in the buffer tank flows intothe etching chamber.
 10. The plasma system as recited in claim 9,further comprising: a pump device for evacuating the etching chamber;wherein the etching gas and the passivation gas are supplied to theetching chamber alternately during separate, independently controlledetching and passivation steps, and wherein a region of the etchingchamber where the plasma source acts on the etching gas is substantiallyfree of the passivation gas during the etching steps, and wherein aregion of the etching chamber where the plasma source acts on thepassivation gas is substantially free of the etching gas during thepassivation steps.
 11. The plasma system as recited in claim 9, whereinthe etching gas line is provided with an etching gas valve forselectively interrupting a supply of the etching gas to the etchingchamber before the passivation gas is supplied to the etching chamber.12. The plasma system as recited in claim 11, wherein at least one ofthe etching gas valve and the passivation gas valve is positioned at adistance of less than 20 cm upstream from the etching chamber.
 13. Theplasma system as recited in claim 11, wherein the etching gas valve andthe passivation gas valve are combined into one changeover valve foralternately connecting the etching gas line and the passivation gas lineto the etching chamber.
 14. The plasma system as recited in claim 11,wherein the etching gas and the passivation gas are used alternatelyduring separate etching steps and passivation steps that are controlledindependently of one another, the passivation gas being supplied to theetching chamber substantially only during the passivation steps, and theetching gas being supplied to the etching chamber substantially onlyduring the etching steps.
 15. The plasma system as recited in claim 9,wherein the buffer tank has a volume of 0.1 L to 1 L.
 16. The plasmasystem as recited in claim 9, wherein the plasma source is aninductively coupled plasma source, and wherein the etching chamber hasan internal diameter of 5 cm to 20 cm at least in a region near theplasma source, whereby when the plasma source is powered by ahigh-voltage generator, a power per area of more than 5 watts/cm² isprovided inside the etching chamber in one of the region near the plasmasource and at a location of the substrate.
 17. The plasma system asrecited in claim 9, further comprising: at least two coils externallyenclosing the etching chamber and positioned one above the other, thetwo coils having current flows in opposite directions, wherein the atleast two coils are provided between the plasma source and thesubstrate.
 18. The plasma system as recited in claim 9, wherein thecommon feed line has a length of less than 20 cm.